What happens inside a gasoline engine every stroke. The Otto cycle shows why higher compression means more power — right up until the fuel knocks.
Otto Cycle (Engine)Live
gasoline engine efficiency
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The Otto cycle idealizes a gasoline engine: adiabatic compression, constant-volume combustion, adiabatic expansion (the power stroke), and exhaust. Its efficiency, η = 1 − 1/r^(γ−1), depends only on the compression ratio. Higher compression means more efficiency — until the fuel pre-ignites and knocks, which is why octane rating matters.
Reading this result: Efficiency (58%) depends only on the 9:1 compression ratio and γ, not on how much fuel you burn — adding heat gives more power at the same efficiency ceiling.
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How it works
The Otto cycle idealizes a spark-ignition engine: adiabatic compression, constant-volume combustion, adiabatic expansion (the power stroke), and exhaust. Its efficiency, η = 1 − 1/r^(γ−1), rises with the compression ratio r. Push too far and the air-fuel mix pre-ignites and knocks — which is what octane rating resists.
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